[Support] Fix error handling in DataExtractor::get[US]LEB128

[labath]

Summary

These functions are documented as not modifying the offset argument if
the extraction fails (just like other DataExtractor functions). However,
while reviewing D63591 we discovered that this is not the case -- if the
function reaches the end of the data buffer, it will just return the
value parsed until that point and set offset to point to the end of the
buffer.

This fixes the functions to act as advertised, and adds a regression
test.

[dblaikie]

Awesome - thanks!

lib/Support/DataExtractor.cpp

uint64_t DataExtractor::getULEB128(uint32_t *offset_ptr) const {
   unsigned shift = 0;
   uint32_t offset = *offset_ptr;
   uint8_t byte = 0;
 
   while (isValidOffset(offset)) {
     byte = Data[offset++];
     result |= uint64_t(byte & 0x7f) << shift;
     shift += 7;
-    if ((byte & 0x80) == 0)
-      break;
-  }
-
+    if ((byte & 0x80) == 0) {
       *offset_ptr = offset;
       return result;
     }
+  }
+  return 0;
+}
 
 int64_t DataExtractor::getSLEB128(uint32_t *offset_ptr) const {
   int64_t result = 0;
   if (Data.empty())
     return 0;
 
   unsigned shift = 0;
   uint32_t offset = *offset_ptr;
   uint8_t byte = 0;
 
   while (isValidOffset(offset)) {
     byte = Data[offset++];
     result |= uint64_t(byte & 0x7f) << shift;
     shift += 7;
-    if ((byte & 0x80) == 0)
-      break;
-  }
-
+    if ((byte & 0x80) == 0) {
       // Sign bit of byte is 2nd high order bit (0x40)
       if (shift < 64 && (byte & 0x40))
         result |= -(1ULL << shift);
 
       *offset_ptr = offset;
       return result;
     }
+  }
+  return 0;
+}

unittests/Support/DataExtractorTest.cpp

TEST(DataExtractorTest, LEB128) {
   offset = 0;
   EXPECT_EQ(42218325750568106ULL, BDE.getULEB128(&offset));
   EXPECT_EQ(8U, offset);
   offset = 0;
   EXPECT_EQ(-29839268287359830LL, BDE.getSLEB128(&offset));
   EXPECT_EQ(8U, offset);
 }
 
+TEST(DataExtractorTest, LEB128_error) {
+  DataExtractor DE(StringRef("\x81"), false, 8);
+  uint32_t Offset = 0;
+  EXPECT_EQ(0U, DE.getULEB128(&Offset));
+  EXPECT_EQ(0U, Offset);
+
+  Offset = 0;
+  EXPECT_EQ(0U, DE.getSLEB128(&Offset));
+  EXPECT_EQ(0U, Offset);
+}
+
 }